This invention relates to a magnetic focusing type cathode ray tube which employs a magnetic yoke for shaping the focusing magnetic field to improve focusing thereof.
Generally, the focusing means for a cathode ray tube is categorized as either an electrostatic focusing type or as a magnetic focusing type. Of these focusing means, the electrostatic focusing type has been more widely used. However, the magnetic focusing type cathode ray tube has better resolution. Further, a higher supply voltage for focusing is not required. Therefore, a power source circuit associated with the magnetic focusing type cathode ray tube may be simplified, and the electrical insulation means with respect to the higher voltage may be also simplified. This brings out the possibility that reliability of the magnetic focusing type cathode ray tube could be improved and thus its production cost reduced. For these reasons, much effort has been recently made to improve the magnetic focusing type cathode ray tube.
The magnetic focusing type cathode ray tube generally employs an electron gun in the magnetic focusing lens system. The electron gun is constructed by a cathode member and a focusing magnetic yoke assembly. In the in-line type electron gun, for example three cathodes for red, blue and green are arranged in an in-line fashion, and a pair of magnetic yokes having electron beam passing holes corresponding to the cathodes are disposed in a face-to-face manner. The pair of magnetic yokes are coupled with a pair of permanent magnets. The permanent magnets are vertically arranged over the central electron beam path. The N pole of the magnet is closer to the cathode side; and the S pole of the magnets is closer to the screen side of the tube. Each magnetic yoke is provided with cylindrical magnetic elements which protrude from the periphery of the electron beam passing holes.
In an electron gun thus constructed, magnetic flux from the N pole of the permanent magnets passes into the cylindrical magnetic element of the yoke closest to the N pole. The magnetic flux then passes through the other cylindrical magnetic elements of the yoke closest to the S pole, and afterwards returns to the S pole of the permanent magnets. In this way, a focusing magnetic field is generated in the magnetic gaps between the cylindrical magnetic elements of the opposite magnet yokes. All together, three focusing magnetic fields are formed to control each of the three electron beams. Ideally, perfect focusing can be attained only by the focusing magnetic fields of the permanent magnets. Actually, however, other external magnetic fields exist in the cathode ray tube. For example, there is a magnetic field directed from the yoke closest to the N pole side, i.e., the cathodesided yoke, to the cathode itself, and there is another magnetic field directed from the screen to the yoke closest to the S pole side, i.e., the screen-sided yoke. Under the influence of such external magnetic fields, the side electron beams, e.g., the beams for red and green phosphor dots are deflected vertically with respect to the beam path.
One of the most important aspects when the magnetic focusing means is employed for the CRT such as a color picture tube having a plurality of electron guns, resides in the convergence of the three electron beams at the center of the screen. As the result of the undesirable deflection, when the three electron beams are concentrated by a ring-like 4-pole magnet mounted around the screen sided neck portion, the beam spot on the screen forms an ellipsoid, thus degradating the focusing quality. cl SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a magnetic focusing type cathode ray tube with a magnetic field shaping yoke assembly for shaping a focusing magnetic field so as to have a proper beam spot.
According to the present invention, a cylindrical yoke portion for each electron gun and a common yoke portion surrounding all beam paths for a plurality of electron beams are used for the magnetic field shaping yoke assembly.
With this arrangement, a focusing magnetic field on the passage area of the three electron beams can be distributed uniformly. The radial direction magnetic field component can be reduced. The disturbance resulting from the convergence of electron beams in the center of the screen can also be reduced. As a result, a better beam spot can be obtained on the screen, and the focusing can be improved.